In modern antenna design, antenna size can be minimized by tuning an antenna according to an active radio frequency (RF) band. Antenna performance is thus optimized to cover only a specific RF band at a time.
Carrier aggregation improves data transmission speeds in the downlink (and more recently in the uplink) by using two RF bands simultaneously. Carrier aggregation requiring simultaneous operation in two frequency bands is problematic for tunable antennas because there needs to be a compromise between the two frequency bands.
One solution is to use passive antennas that cover the required RF bands. Such an antenna, however, would need to be large, thereby competing with customer demands for the wireless device to be as small as possible.